1. Field of the Invention
The present invention relates to a receiver, a transmitter, a communication system, and a method of communication.
2. Description of the Related Art
The multi-carrier code division multiple access (CDMA) transmission method was proposed in 1993 in MULTI-CARRIER CDMA IN INDOOR WIRELESS RADIO NETWORKS (N. Yee et al., 1993 IEEE Personal, Indoor and Mobile Radio Communications), and studies have been made regarding the application of the same to mobile communication systems since then.
The multi-carrier CDMA transmission method is a method of transmission in which a data symbol is copied in the direction of a frequency axis; each of the copied data symbols is multiplied by one chip of a spreading code and spread; and the spread data signals are transmitted in parallel over a plurality of sub-carriers having different frequencies.
The multi-carrier CDMA transmission method makes it possible to transmit a plurality of data symbols simultaneously. According to the multi-carrier CDMA transmission method, a data symbol is multiplied by a spreading code in the direction of a frequency axis. Therefore, according to the multi-carrier CDMA transmission method, a plurality of data symbols can be code-division-multiplexed by multiplying the data symbols by a spreading code orthogonal to each other.
Further, a low symbol rate and a great symbol length are achieved by the multi-carrier CDMA method because parallel transmission is performed using a plurality of sub-carriers. As a result, the multi-carrier CDMA transmission method makes it possible to reduce the influence of so-called multipath interference that is problematic in a mobile communication environment. Multipath interference is interference which occurs between transmission signals when they arrive at a receiver at different timing via a plurality of different paths (a multipath) and which results in the degradation of signal transmission characteristics. In a multipath, frequency-selective fading occurs in which variations in the path occurs depending on frequency, the signal transmission characteristic varying depending on its frequency. According to the multi-carrier CDMA transmission method, a data signal is spread in the direction of a frequency axis. Therefore, according to the multi-carrier CDMA transmission method, a frequency diversity effect reduces the influence of variation in signal transmission characteristic, thus improving signal transmission characteristics. The multi-carrier CDMA transmission method has many advantages as thus described.
However, the multi-carrier CDMA transmission method has the following problems. As shown in FIGS. 1A and 1B, a receiver receives signals that are signals over a data channel #1 and a data channel #2 multiplexed and transmitted. The receiver multiplies the reception signals by a spreading code in the direction of frequencies, the spreading code being identical to a spreading code that has been multiplied at the transmitter. Then, the receiver performs despreading by combining the reception signals of each sub-carrier over the spreading code duration of the spreading codes.
As shown in FIG. 1A, when the propagation path variation value of the sub-carriers has a constant, the spreading codes multiplied on the data symbols in the respective data channels are orthogonal to each other. Therefore, the data symbols in the respective data channels #1 and #2 are completely recovered in the reception signals after despreading. In practice, however, since the sub-carriers are subjected to different amplitude variations and phase variations as shown in FIG. 1B, the propagation path variation value will not have a constant. Therefore, the orthogonality of the spreading codes multiplied on the reception signals in the respective data channels received after propagating on the multipath is destructed. As a result, the data symbols in the respective data channels #1 and #2 can be not completely recovered from the reception signals after despreading, and the data symbols in the data channels interfere with each other and remain, which degrades signal transmission characteristics.
In order to solve such a problem, a method in which a receiver multiplies a reception signal over each sub-carrier by weights and combines signals to reduce interference between data symbols in data channels has been proposed in Design and Performance of Multicarrier CDMA System in Frequency-Selective Rayleigh Fading Channels (S. Hara et al., IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, pp. 1584–1595, VLO. 48, NO. 5, September 1999), for example.
Diversity combining is a technique for reducing the influence of variation in signal transmission characteristics depending on frequency by the above-described effect of frequency-selective fading, and improving signal transmission characteristics. One scheme for diversity combining is antenna diversity combining in which signals are received by a plurality of antennas and in which combining is performed with the signal received of each of the antennas multiplied by weights. For example, a method for weighting in antenna diversity combining has been proposed in Linear Diversity Combining Techniques (D. G. Brennan, Proc. IRE, pp. 1075–1102, VOL. 47, NO. 6, June 1959).
A proposal has recently been made in which antenna diversity combining is applied to the orthogonal frequency division multiplexing (OFDM) multi-carrier transmission method (CO-CHANNEL INTERFERENCE SUPPRESSION ASSISTED ADAPTIVE OFDM IN INTERFERENCE LIMITED ENVIRONMENTS (M. Munster et al., IEEE Vehicular Technology Conference-Fall, 1999)).
However, studies have been made only on the application of antenna diversity to the OFDM multi-carrier transmission method, and no study has been made at all on the application of antenna diversity combining in the multi-carrier CDMA transmission method.